1. Field of the Invention
The present invention relates to integrated circuit memory elements, and more specifically, to fabrication of capacitors utilizing electrical ceramic oxides, such as ferroelectric materials, in such memory elements.
2. Discussion of the Prior Art
The use of ferroelectric materials to build memory cells is generally known. See U.S. Pat. No. 2,695,396 (Anderson). A ferroelectric material (FE) is "a crystalline substance that exhibits spontaneous electric polarization, electric hysteresis, and piezoelectricity." Webster's Seventh New Collegiate Dictionary, p. 308 (1970).
In a typical ferroelectric cell application, capacitors are built having ferroelectric material sandwiched between a pair of electrodes. The ferroelectric polarization characteristic can be represented by a hysteresis loop (polarization vs. applied energy). Therefore, in a steady state condition (no applied energy) the capacitor will be polarized in one direction (representing, for example, a logical 0). The application of an electric field causes a small displacement in the ion orientation of the crystal structure, thus reversing the polarization of the crystal (logical 1).
Ferroelectric capacitors have been combined with integrated circuit technology to provide non-volatile memory devices. See U.S. Pat. No. 4,149,301 (Cook). For example, ferroelectric capacitors have been used in conjunction with dynamic random access memories (DRAM's) to store and refresh a volatile memory cell in a power interrupt mode, sometimes called a "shadow" RAM.
A variety of electrical ceramic oxides exist, such as those used as ferroelectric capacitors for integrated circuit memories (lead titanate, PbTiO.sub.3 ; lead zirconate titanate, "PZT"; lanthanum doped PZT, "PLZT"; and barium titanate, BaTiO.sub.3). Electrical ceramic oxides are also used in electro-optical devices ("PLZT"; lithium niobate, LiNbO.sub.3 ; and bismuth titanate, Bi.sub.4 Ti.sub.3 O.sub.12) and high temperature superconductors (yttrium barium copper oxide, YBa.sub.2 Cu.sub.3 O.sub.7). The properties of these electrical ceramic oxides are typically optimized by heat treatments in oxidizing ambients at high temperatures (for example, 500.degree. C. to 1100.degree. C.). Many common materials are not suitable for use under such conditions. For example, aluminum melts or reacts with the electrical ceramic oxide material, while tungsten and molybdenum are destructively oxidized. Silicides and polysilicon either react with the electrical ceramic oxides at high temperature or are oxidized at the surface in contact with the electrical ceramic oxide. Silicon dioxide and silicon nitride may also react at these higher temperatures. For these reasons, it has been difficult to build rugged and reliable ferroelectric capacitors on a miniaturized scale appropriate for IC requirements. It has been recognized that homogeneity and uniformity of the ferroelectric region between capacitor plates is important to the ability of the capacitor to retain data (maintain polarization state) as the size of the device gets smaller.
Thus, it would be desirable if a reliable memory cell using a ferroelectric capacitor could be built in the course of integrated circuit fabrication, such that IC operation would be unaffected by the ferroelectric process.